Increasing concerns about biological hazards of minute quantities of lead have recently resulted in downward revision of national standards for toxicity levels and have introduced simultaneously a need for sensitive biomarkers of very low body burden. Pyrimidine 5'-nucleotidase is extremely susceptible to heavy-metal inactivation, especially when cmopared to a companion erythrocyte isozyme, deoxyribonucleotidase. Preliminary studies indicate that these isozymes might provided useful indicators of biological burdens equivalent to 10-20 ug lead/dl of whole blood or less. The objective of this proposal seeks to establish the validity of using two specific erythrocyte enzyme measurements as sensitive indicators of very low body burdens of lead overburden inhumans. This proposal seeks to establish the validity of using two specific erythrocyte enzyme measurements as sensitive indicators of very low body burdens of lead and other heavy metals, particularly candmium and mercury. The hypothesis is that there will be no difference between the currently accepted practice of quantitative assays of whole blood lead and the use of two specific erythrocyte enzyme measurements as sensitive indicators of very low body burdens of lead and other heavy metals, particularly cadmium and mercury.